In vehicles, a change gear mechanism exists between an internal combustion engine and the driving wheels. The change gear mechanism functions to change driving forces applied to the driving wheels and to change a vehicle speed in accord with running conditions of the vehicle, which running conditions change within a wide range, thereby optimizing the performance of the internal combustion engine. As one form of change gear mechanism, for example, there is a continuously variable speed change gear mechanism in which a pulley includes a fixed pulley member fixed to a rotary shaft and a movable pulley member supported on the rotary shaft so as to be movable axially toward and away from the fixed pulley member under oil pressure control so that a width between the fixed and movable pulley members of this pulley is decreased or increased, thereby reducing or increasing the effective rotational radius for a belt extending around the pulley, so that a motive power is propagated, and so that a change gear ratio (or belt ratio) is changed. This type of continuously variable speed change gear mechanism has been disclosed in, for instance, Japanese Publication Nos. 57-186656, 59-43249, 59-77159, and 61-233256.
In effecting control of the continuously variable speed change gear mechanism so that the change gear ratio is changed using oil pressure as mentioned above, the line pressure of the hydraulic circuit, the primary pressure which acts on the movable pulley member in order to actually change the change gear ratio, and the clutch pressure which acts on the hydraulic clutch are classified into five kinds of control modes, including a neutral mode, a hold mode, a normal start mode, a special start mode, and a driving mode, as shown in Table 1. These pressures are variously controlled by open loop control, closed loop control, and/or a duty ratio in which an output value is constant.
TABLE 1 ______________________________________ Control mode and duty output Control Normal Special Mode Neutral Hold Start Start Drive ______________________________________ Line 0% 25% Open Open Closed Loop Loop Loop 25-95% 25-95% 5-95% Ratio 0% Or 0% Or 0% Closed Closed Closed Closed Loop Loop Loop Loop 5-95% 5-95% Clutch 100% Closed Closed Closed 0% Loop Loop Loop 5-95% 5-95% 5-95% ______________________________________ OPEN LOOP: Pressure, rotational speed, etc. are not fed back. CLOSED LOOP: Pressure, rotational speed, and ratio are fed back.
A pressure control valve arrangement which includes an electromagnetic valve and the like for controlling a frequency of the duty output value, and thus an oil pressure of the hydraulic circuit, is driven at a relatively high frequency of 100 Hz in order to smooth the control characteristic of the hydraulic clutch, in particular in order to suppress and lower the vibration which is generated when the hydraulic clutch is slid and coupled.
However, the viscosity of oil in the hydraulic circuit is high at low temperatures (for example, -10.degree. C. or less). Therefore, when a driving frequency of the pressure control valve arrangement is set to 100 Hz, it is impossible at low temperatures to obtain an output pressure of the electromagnetic valve sufficient to drive a change gear control valve of the pressure control valve arrangement. Therefore, even if a driver tries to start movement of the vehicle, the clutch pressure and the like are not set to proper values, so that there occurs an inconvenience in that the connection of the hydraulic clutch becomes defective and the vehicle movement cannot be smoothly started.
However, when the pressure control valve arrangement is driven at the driving frequency of 100 Hz, there occurs an inconvenience in that the durability of the pressure control valve arrangement deteriorates.
On the other hand, if the driving frequency is switched to the frequency of 50 Hz in a low speed running mode at ordinary or high temperatures, a pressure in the hydraulic circuit fluctuates so that there occurs an inconvenience in that this pressure fluctuation results in vibration of the vehicle body and imparts an unpleasant feeling to the driver and passengers.
On the other hand, there is known a continuous variable speed change gear having a hydraulic clutch for intermittently connecting and disconnecting a motive power by an oil pressure. This hydraulic clutch is controlled in various kinds of control modes on the basis of the signals indicative of the engine speed, opening degree of a throttle valve of a carburetor, and the like.
In the conventional line pressure control method, when a vehicle is first actuated (or in other words moved) after the engine starting operation was performed at low or super low temperatures, there is an inconvenience such that the line pressure does not rise as shown in FIG. 15.
Therefore, the clutch pressure does not reach a necessary value and the clutch control cannot be performed, so that the engine speed increases too high and the engine torque is not transferred to the wheels. The clutch is not come into engagement. Finally, there is an inconvenience such that the operating mode cannot be shifted from the normal starting mode.
On the other hand, as shown in FIG. 16, in the conventional line pressure control in the normal starting mode, an object line pressure (P.sub.LINSP or P.sub.LIN) is determined in accordance with the opening degree of a throttle valve. A line pressure solenoid drive duty (D.sub.LIN) which is necessary to obtain the object line pressure is read out of a map (OLSCHD). thereby performing the open loop control to drive a solenoid on the basis of the drive duty ratio. At this time, the feedback control of the line pressure is not executed.
Therefore, by controlling the line pressure by the open loop control, the insufficient increase in line pressure at low temperatures cannot be corrected and the clutch control cannot be performed. Thus, there are inconveniences such that the drive feeling at low temperatures deteriorates, a chain slip occurs in the driving mode, and the safety deteriorates.